This application claims priority to Japanese Patent Application No. P2000-133863 filed May 2, 2000 in Japan and Japanese Patent Application No. P2001-088555 filed Mar. 26, 2001 in Japan. The contents of the aforementioned applications are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a fuel cell. More specifically, the present invention relates to a fuel cell assembled such that a membrane electrode assembly, which is formed by a solid polymer electrolyte membrane and an anode side gas diffusion electrode and a cathode side gas diffusion electrode, is held by a pair of separators. Especially, the present invention relates to a fuel cell in which the membrane electrode assembly is sealed with certainty between the separators.
2. Description of Related Art
In conventional fuel cells, the membrane electrode assembly comprises a solid polymer electrolyte membrane, and an anode side diffusion electrode and a cathode side diffusion electrode which are located at both sides of the membrane. The membrane electrode assembly is held by a pair of separators. By supplying fuel gas (for example, hydrogen gas) onto a reaction surface of the anode side diffusion electrode, the hydrogen gas becomes ionized, and moves toward the cathode side diffusion electrode through the solid polymer electrolyte membrane. The electrons produced in this process flow through an external circuit, and can provide electric energy in the form of a direct current. Since an oxidizing gas (for example, air which contains oxygen) is supplied to the anode electrode, water is generated by the reaction of the hydrogen ions, the electrons, and the oxygen.
One example of a conventional fuel cell is explained with reference to FIG. 20. In FIG. 20, reference numeral 1 denotes the solid polymer electrolyte membrane. A fuel cell 4 is assembled such that the solid polymer electrolyte membrane 1 is held between gas diffusion electrodes (an anode side diffusion electrode and a cathode side diffusion electrode) 2 and 3. A pair of separators 5 is provided so as to sandwich the fuel cell, and an O-ring 7 is fit to a groove portion 6 formed on each of the separators 5. Thus, the solid polymer electrolyte membrane 1 is held by the O-ring 7 and, in that state, the fuel cell 4 is held between the separators 5 (refer to Japanese Unexamined Patent Application, First Publication No. Hei 8-148169).
In the above conventional fuel cell, the O-ring 7 separates the spaces between the separators 5 and the gas diffusion electrodes 2 and 3 from the outside. Therefore, this fuel cell advantageously prevents the leakage of the fuel gas and the oxidant gas, and prevents the mixing of those gases, to thereby achieve efficient electric power generation. However, even a slight shift in the position of the O-ring 7 may result in an insufficient seal reaction force and deteriorate the sealing property thereof. Also, if the solid polymer electrolyte membrane is pulled in the vertical direction in FIG. 20 and twisted due to the above-mentioned shift in the position of the O-ring 7, a force separating the solid polymer electrolyte membrane 1 and the gas diffusion electrodes 2 and 3 may be generated and this phenomenon is not preferable.
In order to avoid the above-mentioned problem, it is necessary to strictly control the accuracy of the size of the groove portion 6. However, this leads to an increase in the manufacturing cost.
Accordingly, an object of the invention is to provide a fuel cell having an improved sealing property between the membrane electrode assembly and the separators, which may be produced readily at a reasonable manufacturing cost.
Accordingly, one of the objectives of the present invention is to provide a fuel cell including a membrane electrode assembly having a solid polymer electrolyte membrane (for instance, a solid polymer electrolyte membrane 18 in the embodiments), an anode side diffusion electrode (for instance, the combination of an anode electrode 22 and a second gas diffusion layer 26 in the embodiments) and a cathode side diffusion electrode (for instance, the combination of a cathode electrode 20 and a first gas diffusion layer 24 in the embodiments) located at both sides of the solid polymer electrolyte membrane, and a pair of separators (for instance, a first separator 14 and a second separator 16 in the embodiments) which holds the membrane electrode assembly. The fuel cell further includes a first seal (for instance, a first seal S1 in the embodiments) substantially disposed between one of the separators and a periphery portion of whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area, the first seal being disposed so as to surround whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the smaller surface area; and a second seal (for instance, a second seal S2 in the embodiments) substantially disposed between the separators so as to surround whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area.
According to the above fuel cell, the first seal and the second seal may function independently of each other. Thus, problems such as deficiencies in the sealing force caused by a positional shift of the seals may be eliminated and, hence, the seal seals the membrane electrode assembly and the separators with certainty. Also, although it is possible to use only members made of the same kind of material in order to equalize the reaction force for the cases where the seals are disposed so as to oppose each other, such an effect of the reaction force need not be considered according to the present invention and the material to be used may be freely selected.
In accordance with another aspect of the invention, the size of the solid polymer electrolyte membrane is smaller than the size of whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area.
According to the above fuel cell, since the size of the solid polymer electrolyte membrane, which is expensive, is decreased, it becomes possible to reduce the cost of the solid polymer electrolyte membrane and the fuel cell per se.
In yet another aspect of the invention, at least one of the first seal and the second seal makes contact with an end face of the anode side diffusion electrode or an end face of the cathode side diffusion electrode.
According to the above fuel cell, since at least one of the first seal and the second seal makes contact with an end face of the anode side diffusion electrode or an end face of the cathode side diffusion electrode, it becomes possible to prevent a reaction gas from leaking out of the end face and from passing through to the outlet side without making contact with the electrically active surface. Accordingly, the seal of the fuel cell may further be improved.
In yet another aspect of the invention, the first seal makes contact with an end face of whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the smaller surface area; and the first seal being extended so as to cover whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area.
According to the above fuel cell, the first seal prevents the reaction gas from leaking out of the end face. Also, since no space is present between the first seal and the second seal, it becomes possible to prevent the reaction gas from passing through to the outlet side without making contact with the electrically active surface. Accordingly, the sealing property of fuel cell may further be improved, and unnecessary pressure is not applied to a sealed portion by an expansion/contraction of the space between the end face and the seal due to changes in temperature.
In yet another aspect of the invention, the second seal makes contact with both an end face of the first seal and an end face of whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area.
According to the above fuel cell, since the first seal and the second seal prevent the reaction gas from leaking out of both of the end faces, it becomes possible to prevent the reaction gas from passing through to the outlet side without making contact with the electrically active surface. Accordingly, the sealing property of fuel cell may further be improved.
In yet another aspect of the invention, the size of the solid polymer electrolyte membrane and the size of whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area are substantially the same.
According to the above fuel cell, the solid polymer electrolyte membrane and whichever of the anode side diffusion electrode and the cathode side diffusion electrode has the larger surface area may be assembled first and then the edge portions thereof may be cut to be flush to each other. Thus, the fuel cell may be readily manufactured.
In yet another aspect of the invention, the anode side diffusion electrode includes an anode electrode, which is a catalyst portion, and a diffusion layer; and the cathode side diffusion electrode includes a cathode electrode, which is a catalyst portion, and a diffusion layer, wherein the size of the catalyst portion of the anode side diffusion electrode and the size of the catalyst portion of the cathode side diffusion electrode are substantially the same.
According to the above fuel cell, since the amount of the catalyst portion, which is expensive, may be decreased, it becomes possible to reduce the manufacturing costs.
In yet another aspect of the invention, the separators are made of dense carbon or a thin metal plate (for instance, a first separator 114 and a second separator 116 in the embodiments).
According to the above fuel cell, for the case where the separators are made of a thin metal plate, they may be produced easily by using a press molding method. Therefore, the manufacturing cost thereof may be reduced and the productivity may be increased.
In yet another aspect of the invention, the first seal (for instance, a first seal S11 in the embodiments) and the second seal (for instance, a second seal S12 in the embodiments) are provided with the same separator.
According to the above fuel cell, since both of the seals may be produced in one process, the number of manufacturing steps may be decreased.
In yet another aspect of the invention, each of the first seal and the second seal is provided with a different separator.
According to the above fuel cell, seals made of different materials may be used for each of the separators which may be made of a metal. Accordingly, the design range of the seals as well as the separators may be widened.
The present invention also provides a fuel cell including a membrane electrode assembly having a solid polymer electrolyte membrane, an anode side diffusion electrode and a cathode side diffusion electrode located at both sides of the solid polymer electrolyte membrane, and a pair of separators which holds the membrane electrode assembly. The fuel cell further includes a first seal substantially disposed between one of the pair of separators and the membrane electrode assembly, and a second seal substantially disposed between the pair of separators so as to be shifted in position outwardly with respect to the position of the first seal to form a double seal together with the first seal.
According to the above fuel cell, a double sealing effect may be obtained with respect to the reaction gas inside the first seal and, hence, safety may be improved by reducing the chance of leakage of the reaction gas.
The present invention also provides a fuel cell including a membrane electrode assembly having a solid polymer electrolyte membrane, an anode side diffusion electrode and a cathode side diffusion electrode each located on a different side of the solid polymer electrolyte membrane, and a pair of separators which holds the membrane electrode assembly. The fuel cell further includes a groove portion provided with the anode side diffusion electrode or the cathode side diffusion electrode, the groove portion being so formed to expose the solid polymer electrolyte membrane, a first seal provided with one of the separators, the first seal being inserted into the groove portion so as to make contact with the solid polymer electrolyte membrane, and a second seal provided with one of the separators, the second seal being shifted in position outwardly with respect to the position of the first seal and making contact with the other one of the separators.
In yet another aspect of the invention, the anode side diffusion electrode and the cathode side diffusion electrode of the above fuel cell are of the same size.
According to the above fuel cell, since the front face of the solid polymer electrolyte membrane may be compressed from both sides, it becomes possible to prevent the generation of cracks in the solid polymer electrolyte membrane even if the water content of the membrane is changed and the membrane is expanded/contracted.
In yet another aspect of the invention, the first seal and the second seal of the above fuel cell are provided with the same separator.
According to the above fuel cell, since both of the seals may be produced in one process, the number of manufacturing steps may be decreased.
In yet another aspect of the invention, each of the first seal and the second seal of the above fuel cell is provided with a different separator.
According to the above fuel cell, seals made of different materials may be used for each of the separators which may be made of a metal. Accordingly, the design range of the seals as well as the separators may be widened.